Sprains of the lateral ligaments of the ankle are one of the most common injuries in sports and recreation. Although many patients with lateral ankle sprains have good clinical outcomes, long term follow-up studies have reported that between 10 to 40% of patients experience chronic symptoms including instability, pain, and osteoarthritis. Abnormal kinematics have been thought to contribute to the development of osteoarthritis, but the precise mechanisms remain unclear. The effects of lateral ankle ligament injury on in vivo joint kinematics and articular contact are not well understood. In an effort to restore normal joint motion and slow the development of osteoarthritis, authors have advocated surgical reconstruction of the lateral ankle ligaments. However, the appropriate surgical treatment of lateral ankle ligament injuries remains controversial, with more than 80 different reconstruction techniques described in the literature. Long-term follow-up studies indicate that there is a high incidence of osteoarthritis after reconstruction of the lateral ankle ligaments. Despite the many techniques described in the literature, the ability of reconstruction to restore normal joint mechanics under functional loading conditions is unknown. The objective of this proposal is to measure the change in in vivo joint kinematics after lateral ankle ligament injuries, using the contralateral ankle joint as a control. The ankle motion of the patients will be evaluated during several weight-bearing activities using both a video gait analysis and an orthogonal fluoroscopic and magnetic resonance imaging technique. In addition, the fluoroscopic and magnetic resonance imaging technique will be used to quantify the effects of lateral ankle ligament injury on magnitude and location of the peak in vivo cartilage contact strains. Finally, the effects of surgical reconstruction on kinematics and contact strains will be measured at 6 and 12 months post-operatively. This study will directly compare the in vivo kinematics and joint contact strains of the injured ankle to that of the intact ankle. These data will help to elucidate the mechanisms contributing to degenerative changes in patients with patients with lateral ankle ligament rupture. Furthermore, the effects of surgical reconstruction on joint mechanics will be evaluated. These data may be used to optimize surgical reconstruction techniques of the lateral ankle ligaments. The appropriate treatment of chronic lateral ankle sprains remains controversial. The aim of this study is to quantify the joint motion and cartilage contact strain distributions of patients with lateral ankle ligament injuries before and after surgical reconstruction. These data will help to identify mechanisms leading to joint degeneration after these injuries and may lead to better reconstruction techniques.